Automatic saturation adjustment

ABSTRACT

A method of correcting image saturation is performed by defining saturated colors in a chromaticity color space; and deriving an overall correction of the saturation of colors in the image using the defined saturation of these colors in conjunction with a table of corrections indexed by hue and lightness. This process has a unique attribute of being able to automatically decrease saturation in regions of an image where oversaturation is detected.

PRIORITY CLAIM

This application is a continuation of U.S. application Ser. No.09/900,441, filed on Jul. 6, 2001 and entitled “Automatic SaturationAdjustment”, incorporated by reference herein for all that it disclosesand teaches.

TECHNICAL FIELD

The present invention relates to the field of digital imaging, and moreparticularly to automatic correction of saturation in color digitalimaging data.

BACKGROUND

With the proliferation of equipment such as digital cameras, scannersand computers digital imagery has become ubiquitous. The world wide webhas also contributed its share of digital imagery. Frequently, whetherbecause of limitations of technology or of skill, images are of lessthan optimal quality. While an expert may have the skill and talent toadjust and correct such images, a consumer without training in the imageprocessing art finds it more difficult. Such a person could benefit fromautomated means of image quality improvement. Even the expert canbenefit from such means since they can be a way of saving time. Amongthe image properties requiring correction is saturation. A properlysaturated image has vibrant or vivid colors, which, however, maintain anatural look. Though it is perhaps more common to encounterundersaturated images, oversaturated images occur with some frequency.Most usually, the latter are the result of misapplied correctionsperformed with image editing software. Any automated method of imagesaturation adjustment must, therefore, cope with both oversaturated andundersaturated images. At present, no such method is available.

European. Pat. 1 017 241, and the references disclosed therein,describes a method of correcting image saturation following a change inthe image tone curve (i.e. image contrast). However, the disclosure isconcerned with restoring the saturation to an appearance similar to thatprior to the change in contrast, rather than with improvement of thesaturation. U.S. Pat. No. 5,982,926 discloses, along with otheroperations, the transformation of image chrominance to correspond to theentire dynamic range of chrominance values. This process cannot correctoversaturated images and, indeed, may oversaturate well adjusted images.U.S. Pat. No. 4,812,902 describes a method of increasing chrominanceusing a non-linear characteristic curve, which has as its objective anincrease in the saturation of low-saturation pixels without an increasein the saturation of highly saturated pixels. Such a process cannotreduce image saturation and, by boosting the saturation of areas of lowchrominance, can contribute to a perception of color imbalance. Thisproblem is addressed in U.S. Pat. No. 5,390,034, which accomplisheschroma correction by taking into account both luminance and chrominanceto determine correction parameters. The parameters are chosen toincrease chrominance where it is high and to leave regions of lowchrominance and luminance unchanged, or even to force their completedesaturation. This recipe does not allow image saturation as a whole tobe lowered. U.S. Pat. No. 5,517,335 describes a method of increasingsaturation of halftone images by removing an increment from the leastintense primary color and adding it to the most intense primary color.Again, such a procedure can only increase the saturation of an imagethough the changes do not correspond well to human perception ofsaturation. A very similar process is described in U.S. Pat. No.5,790,282, wherein the color increment can be of both positive andnegative sign, so permitting desaturation. However, this patent does notteach any means of automatically adjusting saturation. A method ofautomatic saturation adjustment is described in a paper entitled“Adaptive Quality Improvement Method for Color Images” (A. Inoue and J.Tajima, NEC Res. Dev., 35, 180 (1994)). Here, the saturation of allimage pixels in the HSV (hue-saturation-value) color space is linearlystretched such that the most saturated pixel acquires the maximumsaturation it is possible to display in this space. This approach canonly increase saturation and is, moreover, not robust since theadjustment depends on the saturation of but a single pixel, which may besubject to noise. U.S. Pat. No. 5,450,217 describes a more reliable andsophisticated variant of this type of correction that is based on anaverage saturation within certain value (i.e., lightness) bounds andcompensation of image luminance for a change induced by the correctionof saturation in the HSV color space. If the average saturation is lessthan a target value it is increased, otherwise it is left unchanged.Accordingly, this method does not provide a means of reducing excessivesaturation. WO 97/01151 claims a “procedure for automatically maximizingthe visual quality of an image in electric format, characterized in thatthe following operations are performed automatically: the image isconverted into a chromaticity coordinate format; the probable graynessbalance is determined for the image by a statistical method from thevideo signal and the grayness balance of the image is adjusted tocorrect it; the color reproduction of the image is adjusted; and themaximized image is converted to from the chromaticity coordinate formatback to its original format”. While the Lsa chromaticity space discussedin the patent includes a saturation coordinate, s, no explicit mentionis made of saturation adjustment. Moreover, the patent does not discloseany specific embodiment of the invention that would enable theadjustment of “colour reproduction”.

Accordingly, there remains a need for a method of automaticallyadjusting the saturation of an image that will reliably provide adesirable saturation whether the initial image is oversaturated orundersaturated.

SUMMARY

It is, therefore, an aspect of this invention to provide a means ofautomatically and appropriately adjusting the saturation of digitalimages, no matter whether the saturation is initially too low or toohigh, or initially correct and, therefore, requiring the existingsaturation to be preserved.

This and other aspects of the invention are achieved by defining a rangeof highly saturated colors in a chromaticity color space and using thesaturation of these colors, in conjunction with a table of correctionsindexed by hue and lightness, to derive an overall correction for thesaturation of colors in the image.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a graphic display of a chroma histogram.

FIG. 2 is a first table relating values of lightness and hue that may beused in conjunction with a process according to the present invention,the values being representative, not necessarily absolute.

FIG. 3 is a second table relating values of lightness and hue that maybe used in conjunction with a process according to the presentinvention, the values being representative, not necessarily absolute.

DETAILED DESCRIPTIONS

This invention defines a system, apparatus, and software that operate ondigital images. A digital image comprises a collection of pictureelements or pixels arranged on a regular grid. A gray scale image isrepresented by a channel of specific brightness values at individualpixel locations. Such a channel may also be represented as a colorpalette, for example, containing 256 shades of gray. Gray scale imagescontain only achromatic colors, in other words those without a specifichue and with a zero saturation. A color image contains several channels,usual three or four, to describe the color at a pixel. For example,there may be red, green and blue (RGB) channels, or cyan, magenta,yellow and black (CMYK) channels. Each channel again contains brightnessvalues representing the amount of color at each pixel. A color image mayalso be represented in palettized form. Such images contain chromaticcolors, which possess both hue and saturation. The invention isconcerned with adjustment of the saturation of such color images. Itdoes not, however, depend in any way on the color representation usedfor the original image.

Regardless of the original color representation of the image, theinvention is practiced in a chromaticity color space. Therefore, anyoriginal image must at some stage be provided in or converted to digitalimage data defined in a chromaticity color space. Such a color space hasthree dimensions, one of which corresponds at least approximately to thehuman perception of a property variously referred to as lightness orbrightness or luminance or luma or value. Orthogonal to this dimensionis a chrominance plane representing the chromatic component of color.Different directions from the lightness axis in the chrominance planecorrespond to different color hues and distance from the lightness axis,at least approximately, related to the perceived saturation or vividnessof the color. Such a color space may be represented in polar coordinatesin which rotation about the lightness axis represents a hue angle orcolor phase, and distance from the hue axis represents saturation. Theterm saturation is often used very generally to mean colorfulness orvividness or strength or vibrancy of a color and this general use isintended here. Colorfulness is defined as the attribute of a visualsensation according to which an area appears to exhibit more or less ofits hue. Formally, saturation is defined as colorfulness relative tobrightness. A related concept is chroma, which is colorfulness relativeto the brightness of a white. Any of these definitions are suitable forthe practice and understanding of this invention.

Typically, images contain regions of various levels of saturation,ranging from vibrant colors to dull or even completely achromatic colors(e.g., grays). These various colors may occupy different sized regionsof the image, so that sometimes dull colors dominate and other timesbright colors dominate. This presents a problem in assessing whatdetermines perception of the overall saturation of the image and,therefore, of the appropriateness of the saturation level. There is ageneral belief that perception of saturation is influenced by thespatial distribution of color but this matter is poorly understood. Itis not clear whether high and low saturation regions need differenttreatment in the course of adjusting saturation. Conventional manualcorrection typically adjusts saturation in a linear fashion whereby allsaturation values are multiplied by the same factor or, alternativelyand less usually, adds a constant offset to the saturation values of theimage. In such a situation the appropriateness of the saturation levelis left to the judgment of the operator. There is no general agreementas to what image characteristic is responsible for an overall all senseof appropriate saturation of an image. For example, Inoue and Tajimaassert in the previously cited reference that the maximum saturationvalue within the image determines the perception of overall saturation.However, U.S. Pat. No. 5,450,217 bases saturation correction on theaverage saturation within a certain value (i.e., lightness) bound. Itis, therefore, a novel aspect of the present invention to provide areliable means of characterizing overall image saturation, especially ameans that permits establishing not only if an image is undersaturatedbut also if an image is oversaturated. A second novel aspect of theinvention is the provision of a method to adjust saturation, includingboth increasing and reducing saturation, that provides consistentresults over a broad range of imagery involving different distributionsof saturation, including both images of properly high or properly lowsaturation. A third aspect of the invention is that bothcharacterization and adjustment of saturation can be conducted in anautomated manner. A fourth significant aspect of the invention is theprovision of a method to adjust saturation according to specific subjectmatter in the image (e.g., skintones, sky colors, grass) eitherautomatically or in a way that is capable of catering to differentpreferences with respect to saturation among various people. These andother aspects of the invention are achieved by defining a range ofsaturated colors, especially highly saturated colors, in a chromaticitycolor space and using the saturation of these colors, in conjunctionwith a table of corrections indexed by hue and lightness, to derive anoverall correction for the saturation of colors in the image. The mannerin which this may be done is explained more fully in the following.

The chromaticity space used for this invention may be a perceptual colorspace, in which small color differences of the same perceptual size arerepresented by approximately the same numerical distance in the colorspace. Examples of such color spaces are color difference systems suchas the CIE L*u*v* and CIE L*a*b* color spaces as described in G.Wyszecki and W. S. Stiles, “Color Science—Concepts and Methods,Quantitative Data and Formulae”, Wiley, New York, 1982. Other colorsuitable color spaces are color appearance systems such as thosedescribed in M. D. Fairchild, “Color Appearance Models”, Prentice-Hall,New York, 1998. Examples include: the Nayatani color model (Y. Nayatani,Color Res. and Appl., 20, 143 (1995)); the Hunt color model (R. W. G.Hunt, Color Res. and Appl., 19, 23 (1994)); the LLAB color model (R.Luo, Proc. SPIE, 2658, 261 (1996)); the RLAB model (M. D. Fairchild,Color Res. and Appl., 21, 338 (1996)); the ZLAB model (M. D. Fairchild,Proceedings of the CIE Expert Symposium '97 on Colour Standards forImage Technology, CIE Pub. x014, 89-94 (1998)); the IPT model (F. Ebner.and M. D. Fairchild, Proc. 6th IS&T/SID Color Imaging Conf., 8 (1998));the ATD model (S. L. Guth, Proc. SPIE, 2414, 12 (1995)); the Grangeradaptation of ATD as disclosed in U.S. Pat. No. 6,005,968; and theCIECAM97s model described in CIE Pub. 131 (1998). Additional usefulcolor spaces include those that take spatial variation of color intoaccount, such as S-CIELAB (X. Zhang and B. A. Wandell, J. Soc.Information Display, 5, 61 (1997)).

Color order systems are color spaces in which colors are arrangedaccording to constant perceptual difference. Such color spaces may alsobe used for the practice of this invention. Examples of such color ordersystems include: the Munsell system (R. S. Berns and F. W. Billmeyer,Color Res. and Appl., 21, 163 (1996)); the Optical Society of AmericaUniform Color Scale (D. L. MacAdam, J. Opt. Soc. Am., 64, 1691 (1974));the Swedish Natural Color System (Swedish Standard SS 0191 02 ColorAtlas, Second Ed., Swedish Standards Institution, Stockholm, 1989);http://www.ncscolour.com/); and the Deutches Institut fir Normung system(M. Richter and K. Witt, Color Res. and Appl., 11, 138 (1984)).

While these perceptual color spaces are generally preferred for thepractice of this invention, other color spaces may also be used. This isbecause the perceptual irregularities in the color space may becompensated by the table-based corrections to be described later.Additional color spaces include those used in television colorstandards, such as YIQ, YUV or YCbCr, the Xerox YES space, the KodakPhotoYCC space, the Tektronix HVC (hue-value-chroma) color space, theHSV (hue-saturation-value) color space described by A. R. Smith, Comput.Graph., 12, 12 (1978) and Lsa color space mentioned in WO 97/01151.Various computer graphics color spaces such as HLS(hue-lightness-saturation), HSL (hue-saturation-lightness), HSI(hue-saturation-intensity) or HSB (hue-saturation-brightness) may alsobe used. These spaces are described in J. Foley, A. van Dam, S. Feinerand J. Hughes, “Computer Graphics: Principles and Practice”, SecondEdition in C, Addison-Wesley, Reading, Mass., 1995.

Of the various color spaces the CIE L*u*v* and CIE L*a*b* color spacesare preferred since they offer sufficient accuracy in a simpleimplementation and are amenable to rapid color transformation from theoriginal image space by use of a look-up table. Of these, CIE L*a*b* isespecially preferred.

High saturation colors within the chromaticity space are preferablydetermined by means of a saturation or chroma histogram. The histogramis a frequency distribution of saturation or chroma for the pixels ofinterest in the image and measures the fraction of image pixels thatfall within specific ranges of saturation or chroma. Such a histogram isillustrated in FIG. 1. High saturation pixels are those that lie in aninterval between the highest saturation pixel in the image, S_(max), andsome lower bound. A preferred lower bound is from about 0.4S_(max) toabout S_(max) and a more preferred lower bound is from about 0.5S_(max)to about 0.9S_(max). An especially preferred lower bound is from about0.5S_(max) to about 0.8S_(max). It is desirable and preferred to furtherrestrict the location of the lower bound within the described limits.One way of doing so is to select as the lower bound that value ofsaturation above which lies a certain specific fraction of the mostsaturated pixels. A suitable fraction is from about 1% to about 20% anda preferred fraction is from about 2% to about 10%. An especiallypreferred fraction is about 5%. If the saturation corresponding to thisfraction lies within the aforementioned limits on the lower bound ofsaturation, then this saturation may be taken as the lower bound of thehigh saturation region. Otherwise, the closer of the two limits to thissaturation may be taken as the lower bound.

In forming the saturation or chroma histogram certain other measures mayadvantageously be taken. Since the contrast sensitivity function of thehuman eye falls off faster at high spatial frequencies for color thanfor lightness it may be advantageous to smooth the chrominancecomponents in the spatial domain of the image. This is because highspatial frequencies in color are invisible to the eye but contributenoise, and hence error, to the histogram. Prior to forming the histogramthe chrominance components may be smoothed by any method well known inthe art, for example a Gaussian filter, and averaging filter or otherlow-pass filter. Suitable filters can be found in X. Zhang and B. A.Wandell, J. Soc. Information Display, 5, 61 (1997) and in the thesis ofMarcus Nadenau, “Integration of Human Color Vision Models into HighQuality Image Compression”, Thesis No.2296, Département D'Électricité,École Polytechnique Fédérale de Lausanne, Switzerland (2000) (availablefrom http://ltswww.epfl.ch/pub_files/nadenau/). Exact adherence to theserecipes is not, however, required as the performance of the invention isrelatively insensitive to this smoothing. For example, a 7 pixel by 7pixel averaging window may be used in place of these specifications.Additionally, it may be desirable to omit from the histogram thosepixels that have extremely low saturation, since these representessentially achromatic colors that do not contribute to the perceptionof saturation. Because image color gamuts narrow at very low or veryhigh lightness, such low saturation colors may, equivalently, be definedaccording to their lightness, since saturation is necessarily low atlightness extremes. It is preferred to exclude from the histogram colorsof low lightness and high lightness, with exclusion of the former beingespecially preferred. Once high saturation colors have been defined inthe histogram, possibly using the additional measures just described,estimation of the saturation correction may be based on these colors.

For every high-saturation color, an optimal value of saturation isconsidered to exist. Here, the term saturation is used generally andincludes both chroma and the strict definition of saturation as chromarelative to lightness. In general, this optimal value depends not juston the initial value of the saturation but also on the hue and lightnessof the color in question. While such an optimal value is mostconveniently and rapidly retrieved from a table such as that shown inFIG. 2, it may also be represented by an equation or equations to besolved dynamically. The table may be constructed in any fashion providedit permits triplets of lightness, hue and saturation to be associatedwith each other. The table may be quantized into any convenientintervals of lightness, hue and saturation. These intervals may beuniform or non-uniform; uniform intervals are, however, preferredbecause accessing the table contents has a lower overhead. Entries inthe table may all be different or some may be identical. The contents ofthe such optimal saturation tables may be derived by analysis of a rangeof well-saturated imagery or by analysis of saturation changes requiredto improve images with poor saturation or both. It is also within thescope of the invention to have multiple tables providing optimalsaturation values. This permits some or all of the colors in the imageto be handled specially, depending on the preferences of the operator.For example, while there may be one table providing an overallimprovement of the saturation of all the image colors, other tables mayprovide more strongly or more weakly saturated colors to match the tasteof the operator. Additionally, tables covering only certain restrictedcolor ranges may be provided. For example, such tables may be tuned toprovide especially excellent saturation for skin tones or to boost thesaturation of sky or grass colors. The latter colors are oftenremembered as more saturated than they are in reality and boosting thesecolors can provide a more pleasing image at the expense of accuracy inscene reproduction. Such tables may be invoked at the operator's requestor automatically. Automatic selection may, for example, be according tothe fraction of pixels in the image that fall within the range of thecolor in question, or on the basis of the location or distribution orshape of the image region containing the color.

With the assistance of the table or tables an optimal saturation may bedefined for each color in the high-saturation region. For any color, i,this relationship may be expressed in the form of a ratio, R_(i), givenby the equation:R _(i) =A[(Optimal Saturation)/(Original Saturation)]+Bwhere A and B are constants. However, the relationship may also beexpressed in other forms, for example such as this:R _(i) =A exp {B[(Optimal Saturation)/(Original Saturation)]+C}where A, B and C are again constants. This latter representation isparticularly preferred since it very significantly decreases erroneouscorrection of certain images at the expense of only slightly sub-optimalcorrection for others.

Such ratios may be used directly to correct the saturation of individualcolors. However, in the interest of improved reliability and robustness,it is preferred to combine these individual color ratios into a singleratio, R. The combination may be accomplished by any statistical methodthat derives a single measure descriptive of a distribution. Forexample, the mode or the median or the average may be used. The averageis especially preferred, so that for n colors in the high saturationregion of the saturation histogram:$R = {\sum\limits_{i = 1}^{n}{R_{i}/n}}$While it is possible to use such a composite value of R directly, it ispreferred to place some additional constraints on the value of R. Forexample, R can be restricted to lie in a certain range to preventexcessive changes in saturation. A preferred range is from about 0.3 toabout 4, and an especially preferred range is from about 0.7 to about 2.The absolute value of the maximum saturation in the image may also betaken into account. This is desirable for preventing unwanted saturationchanges in nearly achromatic images such as, for instance, sepia tonephotographs or pastel wallpaper samples. In such a case, if the maximumsaturation is less than a threshold, the value of R can be set to apredetermined number, for instance unity. A preferred thresholdcorresponds to a CIE L*a*b* chroma of about 10.

The constrained value of R can be used directly to adjust saturation,for example as a multiplier for the saturation values of all the imagepixels of interest. This is a suitable approach when completelyautomatic operation is desired. However, when it is desired to providefor some operator interaction, for example to express differentpreferences or tastes in image appearance, an alternative formulationmay be used. One illustrative embodiment is to express the saturationcorrection factor, F, as:F=G×R+Hwhere G is a multiplier and H is an offset, with one or both of G and Hconnected to operator-adjustable controls. F is a correction factor thatmultiplies the original saturation of the pixels of interest in theimage. H may be thought of as providing a consistent bias towards highersaturation when positive and towards lower saturation when negative. Gcan be considered as strengthening the algorithmic correction if greaterthan unity and weakening it if less than unity. Other relatedembodiments will be evident to those skilled in the art.

Another aspect of the invention may be practiced as follows. Havingselected an image to work the process of the invention upon, the programautomatically detects the presence of predetermined (i.e., default)color ranges, such as, sky colors, grass colors, skintone colors, etc.as described above. All colors except these specified color ranges arecorrected using the “normal” color table, while colors in (at least oneof) these sub-regions (either automatically or operator selectableranges) are corrected according to a table specific to the range.Typically, in the case of ordinary consumers, the intent in thecorrection to be provided by the table may be to oversaturate the color.For example, in the practice of this method, the program can be designedso that if skintones are detected, then only skintones are correctedwithin the image and no other colors are modified. The method cantherefore be practiced wherein at least one color sub-range from therange of colors being corrected is automatically selected and correctedaccording to an associated table while at least some remaining colors inthe range are corrected according to a second table.

Heretofore, the saturation correction has been described as a linearcorrection. Thus, when all the original saturation values are multipliedby the same factor the proportions between individual pixel saturationvalues are maintained after correction. However, the inventorsappreciate that correction may also be performed in a non-linearfashion. This is particularly advantageous in the case of oversaturatedimages. In such images, the distinction between high saturation valueshas been eliminated by clipping at the image color gamut boundary andthe variation of saturation within high-saturation regions has beenpermanently lost. When the high saturation of such images is correctlyreduced, the image may nonetheless have an unnatural look. In such asituation the new saturation, S′_(i), may be calculated from theoriginal saturation, S_(i), according to:S′ _(i) =F(S _(i))^(γ)

Positive values of the exponent γ that are greater than unity causeexpansion of the high-saturation region in proportion to the lowsaturation region. Positive values less than unity cause expansion ofthe low-saturation region and are preferred for oversaturated images.For a γ of unity the correction is linear. Other functional forms mayalso be adopted to handle specific image problems, for examplemathematical functions that within at least a portion of their rangecreate an S-shaped curve.

The invention is illustrated with respect to a specific embodiment.However, it will be appreciated that wide variation is possible withinthe scope of the invention as required by application needs so that thisembodiment is exemplary and not limiting. For clarity, the embodiment isdescribed as a series of steps. It will be understood by those skilledin the art, however, that the order of these steps may be changed andthat steps may either be combined together into a single step or brokeninto a number of separate steps.

Step 1—Preparation of color information. The image is received as orconverted to RGB color channels and converted into the CIE L*a*b*channels using a look-up table. Hue values, H*, are calculated, by wayof non-limiting example only, according to:H*=180 arctan(b*/a*)/π; if H*<0, H*=H*+360and chroma values, C*, are calculated, by way of non-limiting exampleonly, according to:C*=([a*] ² +[b*] ²)^(0.5)The chroma channel is smoothed with a 7 pixel by 7 pixel averagingwindow unless the image is smaller than the size of this filter kernel,in which case no smoothing is performed.

Step 2—Chroma histogram formation. For RGB channel pre-compensation witha gamma of 2.2 and for a D65 white point, the maximum chroma anywhere inthe RGB gamut is 134. Accordingly, chroma histograms are constructedover a range of 0 to 134 in chroma using a 1 chroma unit bin size. Twohistograms are constructed—a normal one and a skin tone histogram. Thenormal histogram is constructed from all chroma values of the pixelsunder consideration for which L* is not less than 15. The skin tonehistogram is constructed from chroma values of the pixels underconsideration that meet these criteria:L*≧155≦C*≦5510≦H*≦70and, therefore, includes only pixels typical of skin tone colors.

Step 3—Definition of high chroma colors. For each of the normal and skintone chroma histograms a region is defined as containing high chromacolors. The upper bound of this region is the maximum chroma value inthe histogram, C*_(max), as illustrated in FIG. 1. The lower boundC*_(low) is located at a chroma value, C*_(5%), that delineates the top5% of chroma values, subject to the constraint that the lower bound liebetween chroma values of 0.5C*_(max) and 0.8C*_(max). ThusC*_(low)=max(C*_(5%), 0.5C*_(max)) and C*_(low)=min(C*_(5%),0.8C*_(max)). Subsequent calculations are performed only on high chromacolors, i.e. those for which C*_(low)≦C*≦C*_(max).

Step 4—Calculation of scale factors. Overall image chroma scale factorsare calculated separately for normal and skin tone colors. These areSF_(N) and SF_(S) respectively. If either of the histograms is empty,the corresponding overall image scale factor is set to unity. Similarly,if C*_(max) for either of these color categories is less than 10, thecorresponding overall image scale factor is set to unity. Otherwise,these scale factors are calculated as follows. For every pixel, i, witha high chroma color, the chroma scale factor SF_(i) may be computedaccording to:SF _(i)=2.3 exp(−0.828C* _(i) /C* _(opt))C*_(opt) is an optimum chroma obtained from a table indexed by lightnessL* and hue H*. This table is shown as FIG. 2 for normal colors and FIG.3 for skin tone colors. The C*_(opt) value is taken as the nearest inthe table to that required, without any interpolation. The individualpixel scale factors, SF_(i), for normal colors and for skin tones areaveraged separately to give overall values, SF_(N) and SF_(S)respectively, for the whole image. If either SF_(N) or SF_(S) liesoutside the range 0.7 to 2.0 it is constrained to lie in this range.

Step 5—Calculation of correction factors. A correction factor for thechroma values of the image is calculated as follows. First, the targetmaximum chroma in the image, C*_(new,max), is computed from:C* _(new,max) =C* _(old,max) ·SF+Offsetwhere is C*_(old,max) is the maximum chroma anywhere in the originalimage and SF is ether SF_(N) or SF_(S), as is explained below. Offset isan absolute offset for the chroma, optionally adjustable by theoperator, with a default value of zero. A correction factor, F, for thechroma values in the image is now computed according to:F=C* _(new,max) /C* _(old,max)For general imagery the use of SF_(N) in estimating the correctionfactor is preferred. However, if the image contains a significant numberof pixels in the skin tone range it is hard to tell, in principle,whether the image is of an oversaturated face or an undersaturated redfire engine. In such a case it is desirable to give the operator anoption to correct for skin tones by using SF_(S) in the estimation of F.When skin tones are being corrected, only colors having L*≧15, 5≦C*≦55,10≦H*≦70 are modified, using C*_(opt) values from the skin tone table inFIG. 3. A suitable threshold for introducing the option of correctingwith SF_(S) is from about 0.5% to about 10% skin tone colors in theimage, with a preferred value of about 1%. Even when the number ofpixels in the skin tone category is less than the threshold it may, forconsistency, be desirable to leave the operator with a skin tonecorrection option. In that case, however, the value of SF_(S) may simplybe estimated as:SF _(S)=0.5 (1+SF _(N))and used for all colors with L*_(i)≧15.

Step 6—Correction of chroma values. The improved chroma value of anypixel in the image, C*_(new,i), in terms of the original chroma value,C*_(i), is given by:C* _(new,i) =C* _(i) ·FIn general the chroma correction afforded by this equation is excellent,particularly for natural scenes on which the tables in FIGS. 2 and 3 areprimarily based. However, to allow for variation of personal taste amongoperators and to handle exotic computer-generated imagery it isdesirable to provide the operator with an optional control to set thevalue of Offset. A suitable range for offset is from about −25 to +25chroma units. A preferred range is from about −17 to about +17 chromaunits. A generally satisfactory and especially preferred range is fromabout −14 to +14 chroma units. An alternative embodiment expresses theoffset as:Offset=Bias+Strengthwhere Bias is chosen from the values −11, 0 and +11 and, independently,Strength is chosen from the values −6, 0 and +6. In such a case, ifeither all Offset values increase chroma or all Offset values reducechroma, it is desirable to ensure there exists at least one combinationof settings that leaves chroma unchanged, preventing undesirable resultswith unusually pathological images. This can be done by recalculatingC*_(new,max) as follows. If C*_(new,max)−Offset is greater thanC*_(old, max), then C*_(new,max) is set toC*_(old,max+Offset; otherwise if C*) _(new,max)+Offset is less thanC*_(old,max), then C*_(new,max) is set to C*_(old,max)−Offset. Finally,a new value of F is computed as in step 4. Following either automaticdetermination of the correction factor F or its adjustment by theopeartor's selection of an Offset value, all the chroma values of thepixels under consideration are corrected to improve values. Then, thenew chroma values are combined with the original L* and H* values of thepixels and converted to L*, a* and b* values. Finally, the image isrestored to RGB values by means of a look-up table.

1. A computer-readable medium storing computer-executable instructionsfor performing a computer process for correcting saturation in a image,the computer process comprising: defining saturated colors in achromaticity color space; and deriving an overall correction of thesaturation of colors in the image using the defined saturation of thesecolors in conjunction with a table of corrections indexed by hue andlightness.
 2. The computer-readable medium of claim 1 wherein thesaturated colors comprise the saturation values of the most saturatedcolors in the image.
 3. The computer-readable medium of claim 1 whereinthe computer process acts upon an image containing data of oversaturatedregions, and the computer process acts to automatically reducesaturation at least where some data of oversaturated regions existwithin the image.
 4. The computer-readable medium of claim 1 wherein acorrection for overall saturation for a specific range of colors isdetermined, and then a correction for a single color within the specificrange is used to direct correction for overall saturation for othercolors.
 5. The computer-readable medium of claim 1 wherein a correctionfor overall saturation for a single color is determined and thatcorrection for a single color is used to direct correction for overallsaturation for other colors.
 6. The computer-readable medium of claim 1wherein a correction for overall saturation for fewer than all colors isdetermined and that correction for fewer than all colors is used todirect correction for overall saturation for other colors.
 7. Acomputer-readable medium storing computer-executable instructions forperforming a computer process for correcting saturation of an image, thecomputer process comprising: defining saturated colors in a chromaticitycolor space; and deriving an overall correction of the saturation ofcolors in the image using the defined saturation of these colors byreferencing a table of corrections relating values of saturation, hueand lightness.
 8. The computer-readable medium of claim 7 wherein valuesof saturation are provided as values of saturation desired for values ofhue and lightness.
 9. A computer-readable medium storingcomputer-executable instructions for performing a computer processcomprising: determining original saturation values of pixels in adigital image; determining target saturation values of the pixelsdependent on at least one of the hue or lightness of the pixels;computing one or more saturation correction factors based on theoriginal saturation values and target saturation values of the pixels;and adjusting the original saturation value of at least some of thepixels toward the target saturation values of the at least some of thepixels by an amount dependent on the one or more saturation correctionfactors.
 10. The computer-readable medium of claim 9 wherein theoperation of determining target saturation values of the pixelscomprises: determining hue values of the pixels; determining the targetsaturation value of each of the pixels based on the hue value of thepixel.
 11. The computer-readable medium of claim 9 wherein the operationof determining target saturation values of the pixels comprises:determining lightness values of the pixels; determining the targetsaturation value of each of the pixels based on the lightness value ofthe pixel.
 12. The computer-readable medium of claim 9 wherein theoperation of determining target saturation values of the pixelscomprises: determining lightness values of the pixels; determining huevalues of the pixels; determining the target saturation value for eachof the pixels based on the lightness value and the hue value of thepixel.
 13. The computer-readable medium of claim 9 wherein the targetsaturation value for a pixel having a first range of colors is differentthan a pixel having a different range of colors.
 14. Thecomputer-readable medium of claim 9 wherein the saturation correctionfactor for a pixel having a first range of colors is different than apixel having a different range of colors.
 15. The computer-readablemedium of claim 9 wherein the adjusting operation increases the originalsaturation value toward the target saturation value for a pixel.
 16. Thecomputer-readable medium of claim 9 wherein the adjusting operationdecreases the original saturation value toward the target saturationvalue for a pixel.
 17. The computer-readable medium of claim 9 whereinthe adjusting operation adjusts the original saturation values of pixelshaving a first range of colors differently than adjustment of theoriginal saturation values of pixels having a different range of colors.18. A method comprising: determining original saturation values ofpixels in a digital image; determining target saturation values of thepixels dependent on at least one of the hue or lightness of the pixels;computing one or more saturation correction factors based on theoriginal saturation values and target saturation values of the pixels;and adjusting the original saturation value of at least some of thepixels toward the target saturation values of the at least some of thepixels by an amount dependent on the one or more saturation correctionfactors.
 19. The method of claim 18 wherein the operation of determiningtarget saturation values of the pixels comprises: determining hue valuesof the pixels; determining the target saturation value of each of thepixels based on the hue value of the pixel.
 20. The method of claim 18wherein the operation of determining target saturation values of thepixels comprises: determining lightness values of the pixels;determining the target saturation value of each of the pixels based onthe lightness value of the pixel.
 21. The method of claim 18 wherein theoperation of determining target saturation values of the pixelscomprises: determining lightness values of the pixels; determining huevalues of the pixels; determining the target saturation value for eachof the pixels based on the lightness value and the hue value of thepixel.
 22. The method of claim 18 wherein the target saturation valuefor a pixel having a first range of colors is different than a pixelhaving a different range of colors.
 23. The method of claim 18 whereinthe saturation correction factor for a pixel having a first range ofcolors is different than a pixel having a different range of colors. 24.The method of claim 18 wherein the adjusting operation increases theoriginal saturation value toward the target saturation value of thepixel.
 25. The method of claim 18 wherein the adjusting operationdecreases the original saturation value toward the target saturationvalue of the pixel.
 26. The method of claim 18 wherein the adjustingoperation adjusts the original saturation values of pixels having afirst range of colors differently than adjustment of the originalsaturation values of pixels having a different range of colors.
 27. Acomputer-readable medium storing computer-executable instructions forperforming a computer process comprising: defining saturated colors in achromaticity color space; and deriving a correction of the saturation ofcolors of pixels in the image using the defined saturations of thesecolors in conjunction with corrections dependent upon at least one ofhue and lightness of the pixels.
 28. The computer-readable medium ofclaim 27 wherein the deriving operation comprises: determining huevalues of the pixels; determining the target saturation value of each ofthe pixels based on the hue value of the pixel.
 29. Thecomputer-readable medium of claim 27 wherein the deriving operationcomprises: determining lightness values of the pixels; determining thetarget saturation value of each of the pixels based on the lightnessvalue of the pixel.
 30. The computer-readable medium of claim 27 whereinthe deriving operation comprises: determining hue values of the pixels;determining lightness values of the pixels; determining the targetsaturation value of each of the pixels based on the hue value and thelightness value of the pixel.
 31. A method comprising: definingsaturated colors in a chromaticity color space; and deriving acorrection of the saturation of colors of pixels in the image using thedefined saturations of these colors in conjunction with correctionsdependent upon at least one of hue and lightness of the pixels.
 32. Themethod of claim 31 wherein the deriving operation comprises: determininghue values of the pixels; determining the target saturation value ofeach of the pixels based on the hue value of the pixel.
 33. The methodof claim 31 wherein the deriving operation comprises: determininglightness values of the pixels; determining the target saturation valueof each of the pixels based on the lightness value of the pixel.
 34. Themethod of claim 31 wherein the deriving operation comprises: determininghue values of the pixels; determining lightness values of the pixels;determining the target saturation value of each of the pixels based onthe hue value and the lightness value of the pixel.